I think it's because people start thinking about what they really want, both immediate gratification-wise and with regards to the bigger picture. If someone asks, "What do you want for Christmas" and the first thought that comes to mind is to be or do something with someone else, or maybe just that they would be so much happier on their own, then that should cause one to rethink their relationship.
And I believe many do. The holidays, with family support, makes it easier because they'll be around other people.
And that's another thing: How many families can't stand the significant other? And vice versa. I wonder how many people think, "What I wouldn't give to not have to spend Thanksgiving and Christmas hating life being around his family."